1. Field of the Invention
This invention relates to a process for separating silicon from silicate-bearing material and, more particularly, to a process involving the carbothermal reduction of silica in a silicate-bearing material to silicon monoxide. The process includes diffusion of the silicon monoxide from the silicate-bearing material where it is reacted with additional carbon to form silicon carbide fibers.
2. The Prior Art
Silicon is one of the most abundant elements in the earth's crust where it is usually combined with oxygen in the form of silica (SiO.sub.2) or as a silicate in a silicate-bearing material. Various silicate-bearing materials are available in suitable quantities for possible exploitation as a raw material source for various products. However, in many instances their usefulness is severely limited by the excessive quantity of silica present. For example, vast deposits of clay (Al.sub.2 O.sub.3.2SiO.sub.2) represent a significant reservoir of alumina (Al.sub.2 O.sub.3), a raw material for the aluminum industry, or even mullite, a useful refractory. Mullite is normally supplied from the mineral kyanite, or artificially by mixing aluminum oxide with clay so as to produce a higher alumina/silica ratio and thereby obtain mullite. Unfortunately, clay, by nature, contains a substantial quantity of silica which interferes with its usefulness as the desired raw material sources for either alumina or mullite.
Mullite (3Al.sub.2 O.sub.3.2SiO.sub.2) is one of the few compounds of aluminum and silicon that is stable at high temperatures. Mullite is, therefore, a useful refractory and does not deform under loads at 1800.degree. C. Mullite is also resistant to corrosion and has a low coefficient of thermal expansion. Unfortunately, mullite is found in nature in only limited deposits while clay is abundantly available but has an excessive silica content which prevents clay from being used as a refractory in place of mullite.
Another silicate-bearing material which could advantageously benefit from the removal of silica is zircon (ZrSiO.sub.4). Removal of the silica would leave zirconia (ZrO.sub.2) which finds many uses as (1) a durable refractory, (2) an effective opacifier of fused enamels, glass, and glazes; (3) the manufacture of Welsbach mantles, and (4) numerous other uses, including the production of zirconium (Zr) which is useful in steel making.
Silicon carbide is another useful product which, historically, has been produced in an electric arc furnace from a reactant mixture including sand (silica, SiO.sub.2) and coke (carbon, C). The reaction occurs at a high temperature and in the vapor phase with a resulting deposition of a non-fibrous, crystalline mass of silicon carbide on the surfaces surrounding the reaction zone. The crystalline mass of silicon carbide is removed from the furnace and treated by further crushing, grinding, and sizing prior to its being used in the abrasives and refractory industries.
However, of recent discovery, it has been found that silicon carbide can be formed also as fibers having a length to diameter ratio greater than about 50:1. The silicon carbide fibers, if produced in sufficient quantity, could serve as a useful reinforcement material for numerous applications in the metalurgical, cermet, ceramic and plastics industries.
In view of the foregoing, it would be a significant advancement in the art to provide a process for removing silicon and, more particularly, slica from silicate-bearing materials. An even further advancement in the art would be to provide a process whereby a controlled quantity of silica could be removed from a silicate-bearing material. Another advancement in the art would be to provide a process for reacting silicon monoxide obtained from a silicate-bearing material with carbon to form silicon carbide fibers. Such a process is disclosed in the present invention.